In the search for high performance materials, considerable interest has been focused upon carbon fibers. Graphite fibers or graphitic carbonaceous fibers are defined herein as fibers which consist essentially of carbon and have a predominant x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern. Graphitic carbonaceous fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and graphitic carbonaceous fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for graphitic carbonaceous fiber reinforced composites include recreational equipment such as golf club shafts, aerospace structural components, rocket motor casing, deep submergence vessels, ablative materials for heat shields on re-entry vehicles, etc.
In the prior art numerous materials have been proposed for use as possible matrices in which graphitic carbonaceous fibers may be incorporated to provide reinforcement and produce a composite article. The matrix material which is selected is commonly resinous in nature (e.g. a thermosetting resinous material) and is commonly selected because of its ability to also withstand highly elevated temperatures.
While it has been possible in the past to provide graphitic carbonaceous fibers of highly desirable strength and modulus characteristics, difficulties have arisen when one attempts to gain the full advantage of such properties in the resulting fiber reinforced composite articles. Such inability to capitalize upon the superior single filament properties of the reinforcing fiber has been traced to inadequate adhesion between the fiber and the matrix in the resulting composite article.
Numerous techniques have been proposed in the past for modifying the fiber properties of a previously formed carbon fiber in order to make possible improved adhesion when present in a composite article. These techniques generally can be classified as either hot gas surface treatments, liquid oxidative surface treatments, or surface coating procedures.
Representative hot gas carbon fiber surface treatments include those disclosed in U.S. Pat. Nos. 3,476,703; 3,723,150; 3,723,607; 3,745,104; and 3,754,957; British Pat. Nos. 1,180,441 and 1,225,005; and Japanese Pat. No. 75-6862. U.S. Pat. No. 3,476,703 and British Pat. No. 1,180,441 disclose heating carbon fibers normally within the range of 350.degree. to 850.degree. C. in a gaseous oxidizing atmosphere such as air for an appreciable period of time. It is there mentioned that an oxygen rich or pure oxygen atmosphere, or an atmosphere containing an oxide of nitrogen may be used. U.S. Pat. No. 3,745,104 discloses a process wherein carbon fibers are subjected to a gaseous mixture of an inert gas and a surface modification gas such as oxygen or nitrogen dioxide in the presence of high frequency electrical power. Japanese Pat. No. 75-6862 discloses treating carbon fibers with a nitrogen monoxide atmosphere.
Representative hot gas plasma treatments are disclosed in U.S. Pat. Nos. 3,767,774; 3,824,398; and 3,872,278.
Representative liquid oxidative surface treatments are disclosed in U.S. Pat. Nos. 3,657,082; 3,671,411; 3,759,805; 3,859,187; and 3,894,884. It generally is essential that the carbon fibers treated in this manner be washed and dried following the liquid oxidative surface treatment.
Representative surface coating procedures are disclosed in U.S. Pat. Nos. 3,762,941 and 3,821,013.
Nevertheless the need has remained for an improved process for the surface modification of carbon fibers which expeditiously can be carried out on an economical basis, while retaining to a substantial degree the tensile strength exhibited prior to the surface treatment.
It is an object of the invention to provide an improved continuous gas phase process for efficiently modifying the surface characteristics of carbon fibers.
It is an object of the invention to provide an improved process for enhancing the ability of carbon fibers to bond to a resinous matrix material.
It is an object of the invention to provide an improved process for modifying the surface characteristics of carbon fibers which may be conducted relatively rapidly and in a controllable manner.
It is an object of the invention to provide an improved process for modifying the surface characteristics of carbon fibers which can be carried out relatively economically without the requirement that a fiber wash step be conducted following the surface modification step.
It is an object of the invention to provide an improved process for modifying the surface characteristics of carbon fibers which has been found to produce a great increase in the surface area of the carbon fibers.
It is an object of the invention to provide an improved process for modifying the surface characteristics of carbon fibers which has been found to be effective with a wide range of carbon fibers of greatly varying Young's moduli levels (e.g. 30 to 80 million psi, or more).
It is an object of the invention to provide an improved process for modifying the surface characteristic of carbon fibers so as to improve their ability to bond to a resinous matrix material while retaining a substantial portion of the tensile strength intact.
It is another object of the invention to provide composite articles exhibiting an improved interlaminar shear strength which are reinforced with the resulting surface modified carbon fibers.
It is a further object of the invention to provide composite articles which are reinforced with the resulting surface modified carbon fibers and exhibit no substantial first failure mode in tensile strength evaluation.
These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.